A head-mounted display, often called a virtual retinal display (VRD), projects an image directly onto the retinas of a viewer's eyes. For example, a VRD may project a transparent image of a flight-instrument panel into a fighter pilot's eyes such that he can see the flight instruments regardless of his viewing direction. Thus, the VRD eliminates the need for the pilot to shift his gaze back and forth between the instrument panel and the view through the windshield.
A VRD typically includes at least one oscillating mirror—often called a micro-electromechanical mirror (MEM)—that scans the image into the viewer's eyes. Typically, the VRD directs an image beam, which is typically a modulated light beam, onto the mirror's reflective face, which directs the beam into one of the viewer's eyes. By oscillating back and forth through a range of horizontal and vertical sweep positions, the mirror sweeps the beam through a raster pattern to scan the image. Alternatively, the VRD may include two oscillating mirrors, one for sweeping the beam in a horizontal-scanning direction and the other for sweeping the beam in a vertical-scanning direction. To scan the image into the viewer's other eye, the VRD may direct a second, identical image beam onto another mirror or pair of mirrors. Alternatively, the VRD may include an optical assembly that splits the swept image beam into a pair of swept beams, one for each eye.
Unfortunately, as discussed below in conjunction with FIG. 1, the forces placed on the mirror while it oscillates may cause the mirror to distort the scanned image to a noticeable and undesirable degree.
FIG. 1 is a view of a conventional scanning-mirror structure 10, which includes a mirror 12 for sweeping an image beam 14 into a viewer's eye (not shown in FIG. 1). In addition to the mirror 12, the structure 10 includes torsion arms 16 and 18 and mounting flanges 20 and 22. The mirror 12 includes a face 24 for reflecting the beam 14 into the viewer's eye. Typically, the mirror 12, torsion arms 16 and 18, and flanges 20 and 22 are formed as an integral unit, and the face 24 is polished and treated with a reflective optical coating.
In operation, the mirror 12 sweeps the image beam 14 in a horizontal-scanning direction by oscillating about an axis 26. Typically, the mirror structure 10 is formed from a magnetic material such as steel and is magnetized, and the flanges 20 and 22 are mounted such that they remain stationary as the mirror 12 oscillates. A conductive coil (not shown in FIG. 1) located near the mirror 12 generates a sinusoidal magnetic field that causes the mirror to rock back and forth about the axis 26 as indicated by the arrows. By setting the frequency of the magnetic field at or near the frequency at which the mirror 12 resonates about the axis 26, one can sweep the beam 14 very efficiently, i.e., with minimum power dissipation in the coil.
Unfortunately, as discussed above, the forces placed on the mirror structure 10 as the mirror 12 oscillates often cause the mirror to distort the scanned image (not shown in FIG. 1) to a noticeable and undesirable degree. Ideally, the mirror face 24 should be flat at all scanning positions through which the mirror 12 oscillates. But as the mirror 12 oscillates, the torsion arms 16 and 18 twist back and forth, and this twisting induces strain in the arms. Typically, the instantaneous twist-induced strain in the arms 16 and 18 at a particular time depends on, e.g., the oscillation frequency, the rotational position of the arms at that time, and the stiffness of the arms. Furthermore, because the mirror 12 has a non-zero mass, it has a moment of inertia about the axis 26. Consequently, each region on the mirror 12 that does not lie on the axis 26 experiences an acceleration, and thus a force, as the mirror oscillates. The forces at these non-axis regions are often high enough to bend the mirror 12, where the degree of bending at a region typically depends on, e.g., the oscillation frequency, the force at the region, and the stiffness of the mirror. Unfortunately, the twist-induced strain in the torsion arms 16 and 18 and the acceleration-induced bending of the mirror 12 often deform the mirror face 24 such that it is not flat at all scanning positions of the mirror. When the face 24 is not flat, it often reflects the beam 14 in a different direction than it would if it were flat. Unfortunately, this shift in the direction of the reflected beam 14 often alters the pattern through which the mirror 12 sweeps the beam 14, and thus often distorts the scanned image.